The dating of old houses... you're in my wheelhouse, now.
Do note that fireplaces can be changed, added, removed... sometimes more than once in the history of an old house. There was a thread on this forum a few years back of one of mine that had been bricked in to look like one of yours, but there were indicators there was an older cooking fireplace hiding behind all the brick work. A few days of removing brick revealed it. Point is, fireplaces may be one data point, but maybe not the best thing for dating an old house.
I agree with
@Mreif that the first two posted look like Rumfords, which if correct would mean they can't be that old. Definitely nowhere near 1720, anyway. Again, newer modifications on older fireplaces? If all-original, then your house is probably 1800 - 1850, and should be much easier to date through various public records.
What county and township are you in? When were each founded? Is there a local historical society? These are always your first places to start.
Look at your floor joists, and even rafters. What is the mix of hand-hewn versus sawn? Are those that are sawn done with a reciprocating saw or circular blade? These things can help narrow down the data range, as floor joists are not as easily replaced as things like windows, and the technology for making them was changing over the years. If only your very long joists are hand-hewn, with shorter ones being milled, you can try to find the longest mill bed available in your area for various dates. Or if they're done with a circular blade, you know the house can't be older than the first circular saw mill in your area. If doing this, don't look at sill plates in barns or any building set on dirt, as those are often replaced.
My current house pre-dates the corporation of both the township and county in which it resides. In those cases, nailing down the date of a house becomes more challenging, unless it was built by someone prominent enough to appear in public records. In my case, the house was built by the minister of one of the largest and earliest churches in our area, so I was able to get a lot of information through that channel. That eventually lead me to family records, which nearly concurred with dates in deeds filed in Philadelphia. Eventually, we were lucky enough to find a date stone hiding under some original stucco, which confirmed some of the dates without doubt, but that's exceptionally rare to find on a house.
This is always a fun project, but in my experience usually takes many years of collecting various info, and many false dead-ends, unless you're lucky enough to actually find mention of a structure listed in a deed. At least in PA, deeds are for land only, and structures are rarely mentioned. When you find any listing of structures on the land, consider it a rare bonus, at least here.
When I moved into my current house, several "experts" told us it was built in 1820-1840, yet I had records indicating it was much older than that. They gave reasons for their insistence on the accuracy of their dates, all of which I've learned were wrong assumptions due to rework done over the years, but it took me many years to figure this out. Our house indeed went thru an absolutely
massive refitting during this time (near 1840), which is why it appears to be that age to so many of these experts on house dating, but we now have concrete and irrefutable proof that an addition was built onto the existing house in 1775. It appears the oldest part of the house probably dates to 1734, based on a combination of public records and many other factors. The "experts" too often claim to be more sure than they should have any right to be, especially when so few of them agree with one-another.
